This invention relates to a process for the preparation of N- and O-substituted low molecular weight aliphatic or aromatic diurethanes and/or polyurethanes in which a primary diamine and/or polyamine is reacted with N-aryl-O-alkyl urethane in the presence of a high boiling alcohol.
The phosgene-free preparation of urethanes and thermal cleavage thereof to form the corresponding isocyanates represents one alternative to preparation of isocyanates by phosgenation of the corresponding amines. Monourethanes which are aromatically substituted on the nitrogen and aliphatically substituted on the oxygen (for example, N-phenyl-O-ethyl-urethane) have been prepared in high yields by various phosgene-free methods. However, known methods for preparing low molecular weight diurethanes and/or polyurethanes which are either aliphatically or aromatically substituted on the nitrogen and aliphatically substituted on the oxygen have serious disadvantages. It is these diurethanes and/or polyurethanes which are technically significant because only they yield the diisocyanates and/or polyisocyanates which are suitable for producing polyurethanes.
The process for the preparation of aromatic di- and/or poly-urethanes from the corresponding amines, urea and alcohol described in German Offenlegungsschrift No. 2,917,569 for example, is disadvantageous in that urea and alcohol are used in large excess so that a high proportion of the urea reacts to form an alkyl carbamate. Since urea and N-unsubstituted carbamates tend to decompose at elevated temperatures, by-products which are non-volatile (such as cyanuric acid, biuret, cyanuric acid triureide, amelide and cyamelide) and which are very difficult to separate from the reaction product may form. Moreover, urea is volatile at high temperatures and forms a solid deposit in the cool parts of the apparatus. Such a solid deposit may cause blockage of pipes and condensers.
The process for the preparation of aliphatic diurethanes and/or polyurethanes described in German Offenlegungsschrift No. 2,917,493 has the disadvantage that considerable quantities of polyurea are formed as by-product. This formation of polyurea is unavoidable since the more highly nucleophilic aliphatic and cycloaliphatic diamines and/or polyamines react much more quickly with urea than the accompanying alcohol. Polyurea, on the other hand, reacts slowly with alcohol to form urethane and amino groups, so that the reaction times are undesirably long.
The processes described in published European Patent Application Nos. 18,581 and 18,583 and German Offenlegungsschrift Nos. 2,917,490 and 2,917,568 are directed to the preparation of aliphatic and aromatic diurethanes and/or polyurethanes from N-unsubstituted carbamic acid esters ("carbonyl source"). These processes have the disadvantage that carbamate must be used in great excess or additional urea must be added. The reaction times described in the examples are very high and the yields are comparatively poor. Such poor yields are attributable to the undesirable formation of by-products, in particular of polyureas.